Call to undefined function: date_default_timezone_set()

[eluser]fatdog[/eluser]

Code:

Fatal error: Call to undefined function: date_default_timezone_set() in /home/my_site/public_html/dev/index.php on line 65

Hi, I'm getting this error.
I have no idea what could be wrong.
The site was working right a few hours ago.
We did a clean reinstall of the site because some repository issues, and now we are getting this.

As far as I know, nothing has changed on the server.

Thanks.